The Obama administration began investigations of hundreds of businesses on Wednesday as part of its strategy to focus immigration enforcement on the employers who hire illegal workers.

The Immigration and Customs Enforcement agency told Congress in an e-mail message that it had begun notifying businesses of plans to audit the documents known as I-9 forms, which employers fill out for every worker. The Department of Homeland Security said 625 businesses had been told they would be audited, compared with 503 for last year.